A neuron in the visual system responds to stimuli that fall within a region referred to as its receptive field. As a consequence of the nerve cell interconnections that subserve receptive field organization, various neurons respond to one type of stimulus better than to another. In the retina of any one species, for example, a limited number of classes of ganglion cells can be defined in terms of the basic properties of their preferred stimuli. This project deals with both the organization and development of the receptive field properties of neurons in the retina and the lateral geniculate nucleus (LGN) of the cat. In the retina of the cat some ganglion cells summate the light falling on their receptive fields in a linear manner and others in a non-linear manner. For a third class of cells this type of characterization is not appropriate. There is evidence that three different morphological classes of ganglion cells correspond to the three physiologically defined ones. If this can be proven, it will enable models to be built that describe the way that individual neurons interact to form the various receptive field types. A direct experiment involving the staining of physiologically identified cells addresses such a proof. In a related series of experiments, the development of these receptive field types will be characterized in kittens to deduce their properties further from the maturational sequences observed. This is possible in kittens because their visual system develops its final form after birth and is amenable to single cell recording. The retinal studies will also be used as a basis for a series of experiments that concern the development of the receptive field properties in the LGN, a thalamic station innervated by retinal ganglion cells. The organization of the LGN is extremely precise in terms of which inputs innervate which target cells. Experiments that deal with how these connections develop can help in our understanding in general terms of how cells in the brain determine their patterns of connections.